Stack Tabs in Opera
You can stack tabs in Opera, just drag one tab on top of another tab, these two tabs are grouped which Opera calls it as a Tab Stack. Opera tries to add something like to Panorama. You can open, close stack with arrow button on last page of the group.
You can unstack all pages by right click on the arrow>Tab Stack>Unstack all pages.
Similarly you can close the stack also by arrow>Tab Stack>Close Stack.
Pinning tabs in Opera
Pinning Tabs in Opera is may not exciting new feature as Firefox 4 dubbed Chrome’s Pinning Tabs as App Tabs, you can pin a tab in Opera by right clicking on the tab and select “Pin Tab”.
You can unpin that tab by right clicking on the tab and uncheck “Pin Tab”. You can Pin a tab, but you can’t pin a tab in stack or stack of Tabs.